(1) Field of the Invention
This invention relates to a mold design system for designing a mold and a computer-readable recording medium storing a mold design program, and more particularly to a mold design system for designing a mold based on three-dimensional CAD data of a product and a computer-readable recording medium storing a mold design program.
(2) Description of the Related Art
Process of molding plastics includes injection molding. In injection molding, a fixed amount of a molten plastic material is injected into an injection mold to mold the plastics into shape. The plastics is thereafter removed from the mold, thus obtaining a product (including a part etc.). When a plastic product or the like is designed, therefore, it is necessary that a mold for making the product should also be designed at the same time. Present-day product design is in most cases carried out by means of CAD (Computer Aided Design), and accordingly, molds are designed based on CAD data of products.
FIG. 34 shows CAD data of a product, by way of example. In the figure is shown a product shape 200 of a square vessel, which is represented by three-dimensional CAD data. This product shape 200 is constructed by a plurality of planes called faces 201. An intersecting line between the faces is an edge 202.
After the above product shape 200 has been completed, a mold surrounding the space of the product shape is designed. Since it is required to remove a molded product, the mold is comprised of at least two upper and lower parts (along a Z axis). The upper part is called a cavity-side part, and the lower part is called a core-side part. In designing the mold, first, a parting line as a boundary between the upper and lower parts is determined. Basically, the parting line is determined with reference to edges which form the outermost peripheral profile of the product. The parting line is determined by designating edges of the product shape, which is carried out by the designer himself. In the illustrated example, a set of edges forming the outermost peripheral profile as viewed along the Z axis forms the parting line 210.
Once the parting line of the product shape is determined, it is possible to cause a computer to calculate profiles of the mold based on CAD data of the product shape 200.
FIG. 35 shows an example of the mold. As shown in the figure, the core-side part 220 is provided with a protrusion in the form of a quadrangular prism, while the cavity-side part 230 is provided with a square hole. An outer periphery of the protrusion of the core-side part 220, and a rim of the hole of the cavity-side part 230 are parting lines 221, 231, respectively.
As described above, by designating the parting line(s) out of the product shape 200 (shown in FIG. 34), it is possible to determine the shape of the mold as shown in FIG. 35.
Now, although it is ideal that a mold is constructed by only two upper and lower parts, it is impossible to avoid existence of an undercut portion if a product shape becomes complicated.
FIG. 36 shows three-dimensional CAD data of a product having an undercut portion, by way of example. This product shape 300 is obtained by forming a hole 310 in a side face portion of the product shape 200 appearing in FIG. 34. The hole 310 is an undercut portion. If such an undercut portion exists, a mold for forming the product shape cannot be constituted by only two upper and lower parts. To overcome this problem, a part called a slide core is employed.
FIG. 37 shows a mold using a slide core, by way of example. As shown in the figure, when an undercut portion exists, the mold includes a slide core 340 in addition to a core-side part 320, and a cavity-side part 330. The cavity-side part 330 is formed with a groove 331 for fitting the slide core 340 therein. After the core-side part 320 and the cavity-side part 330 have been put together, the slide core 340 is fit in the groove 331 of the cavity-side part 330, and in this state, a molten material is injected into the mold and allowed to set or cure. Then, by vertically setting apart the core-side part 320 and the cavity-side part 330, and drawing out the slide core 340 in a direction indicated by an arrow in the figure, the molded article can be removed.
As described hereinabove, by using a slide core, it is possible to remove a molded article even if the product has an undercut portion. However, the use of a lot of slide cores increases the number of parts of a mold, and complicates the process of manufacturing the product, resulting in degraded productivity. To eliminate the inconvenience, in designing a mold, parting lines are determined such that as few undercut portions as possible are produced.
However, when the shape of a product becomes intricate, it becomes very difficult for the user of the CAD system to designate parting lines with accuracy.
FIG. 38 shows a two-dimensional representation of a product shape, as appearing on a display screen in which the product shape 410 is shown as viewed from the direction of the Z axis (mold opening direction). Although in this screen, the outermost periphery of the product shape can be discriminated, it often happens that there are a plurality of edges on the outermost periphery of a product shape. Therefore, in order to designate a parting line, it is required to select one of the edges located on the outermost periphery. Therefore, it is necessary to display shapes of the product as viewed from the directions of an X axis and a Y axis on the screen.
FIG. 39 shows a lower right portion of the FIG. 38 product shape as viewed in the direction of the Y axis indicated by an arrow in FIG. 38. In this figure, it is possible to distinguish edges from each other which overlap in the FIG. 38 screen. From this view, however, it is impossible to discriminate which edge is located on the outermost periphery of the product shape.
As described above, when a product shape is displayed based on two-dimensional CAD data, it is difficult for the user to designate a parting line without an error.
FIG. 40 shows a three-dimensionally displayed product shape. As shown in the figure, if a product shape 420 is three-dimensionally displayed, the product can be recognized stereoscopically. Therefore, to designate a parting line, a portion 421 corresponding to a corner of the product shape 420 is enlarged.
FIG. 41 shows part of the FIG. 40 product shape on enlarged scale. If the product shape is enlarged as shown in the figure, it becomes easy to designate an edge which should be set to a parting line. In this screen, however, it is difficult to accurately determine which edge belongs to the outermost periphery. That is, when there is only a slight displacement between the outermost edge and the other edges, it is impossible to accurately determine which edge is the outermost one.
Therefore, even if a product shape is three-dimensionally displayed, it is difficult for the user to designate a parting line with accuracy.
As will be understood from the discussion stated above, the conventional mold design method tends to give rise to designation of wrong parting lines, and demands a very time-consuming work for the designation. Moreover, with diversification of designs and complication of product shapes as well as frequent model changes of products, in recent years, there is a demand of shorter delivery dates of mold designs. To meet the demand, it is necessary to simplify and speed up the process of mold design, and thereby allow molds to be designed smoothly.